Interference lithography techniques utilize both interference and diffraction characteristics of light to modulate a light-intensity distribution with a particular combination of light beams in an interference field. The light-intensity distribution can be recorded in a light-sensitive material and thereby form a lithography pattern.
Because of the advantages of high resolutions (relatively easily extendable to a quarter of the exposure wavelength) and large depth of focus (nearly equating the coherence length of the light source), interference lithography can be used in periodic patterning of dimensions of several dozens of nanometers to several micrometers.
Currently, devices incorporating periodic patterns, for example, such as long gratings, patterned sapphire substrate, photonic crystals, solar absorbers, field emission displays (FEDs) and so forth require the formation of an array of periodic patterns, either one-dimensional or two-dimensional, uniformly distributed in a high density on two to six inches sized substrates. A key technical problem is how to make sure that the feature patterns be distributed uniformly (i.e., with a very high stitching accuracy) on a substrate with a relative large area.
One of the conventional solutions is provided in U.S. Pat. No. 6,285,817, which describes the invention of a spherical-wave-based interference lithography method using multiple mutually interfering optical beams, wherein several point light sources disposed distal from a target substrate generate spherical waves which propagate and converge on photoresist coated on the substrate to form an interference pattern thereon over a large area. However, due to inherent wave-front distortions of the spherical waves themselves, there is an inconsistency between feature distributions around the center and along the periphery of the substrate.
Another one of the conventional solutions is provided in U.S. Pat. No. 7,561,252, which disclosed a “scan beam interference lithography (SBIL)” method, also referred to as the Doppler writing method. Although this method is capable of fabricating a long grating with a line width as narrow as 100 nm and a remarkably uniform feature distribution on a substrate sized as large as 12 inches, it is unduly complicated and thus cannot be used in the patterning of two-dimensional features.
A third one of the conventional solutions is provided in U.S. Pat. No. 6,882,477 and the literature “Immersion Microlithography at 193 nm with a Talbot Prism Interferometer”, Proc. SPIE 5377, both of which described an interference lithography method using plane waves with wave-front distortions much smaller than those of the spherical waves. However, when this method is used in applications with large exposure fields, phase aberrations caused by various optical members and the environmental media will affect the uniformity of the exposure patterns. While the problem could be alleviated by reducing the exposure field and exposing the substrate field by field, this will cause another problem: as the orientation of the interference exposure patterns is different from the direction of the movement of the wafer stage, there exists an angle α (see FIG. 1) between the coherent interference light beams and the direction of movement of the stage, which can lead to stitching errors across the whole exposure field, as shown in FIG. 2.
As noted above, although there have been developed a number of lithography techniques based on light beam interference, all these techniques are limited for the critical applications where the patterning of periodic micrometer-scaled or nanometer-scaled features in a large area is needed.